1. Field of the Invention
This invention relates to structures suitable for forming capacitors. More particularly, this invention pertains to capacitors embedded within printed circuit boards or other microelectronic devices. The capacitor comprises a pair of parallel electrically conductive foils separated by a pair of coatings of a thermosetting polymer and a central polymerizable layer, each of which preferably contain a filler material. The capacitor exhibits excellent capacitance density and short circuit resistance.
2. Description of the Related Art
A capacitor is a device used for introducing capacitance into a circuit, and functions primarily to store electrical energy, block the flow of direct current, or permit the flow of alternating current. They generally comprise a dielectric material sandwiched between two electrically conductive metal layers, such as copper foils. Such dielectric materials may be coupled to the electrically conductive metal layers via an adhesive layer, by lamination, by coating or by other forms of deposition, e.g. vapor deposition. U.S. Pat. No. 5,155,655 describes one method for forming a capacitor wherein a single sheet of a dielectric material is laminated together with two conductive foils. To optimize the performance of a capacitor, it is important that the dielectric material employed has good material properties exhibiting qualities such as superior adhesion, high dielectric strength and good flexibility.
Capacitors are common elements on printed circuit boards and other microelectronic devices. The capacitor is electrically connected either as a discrete element on the circuit board or may be embedded within the circuit board. Of these options, it has been preferred to form printed circuit boards having embedded capacitors to maximize the surface area of the circuit board for other purposes. The capacitance of a capacitor depends primarily on the shape and size of the capacitor layers and the dielectric constant of the insulating material. There are various known types of dielectric materials known in the art. For example, the dielectric material may be a gas, such as air, a vacuum, a liquid, a solid or a combination thereof. Each material has its own particular properties.
In forming capacitors for use in printed circuit boards, a dielectric material such as a glass reinforced polymer matrix has been used. However, the performance of capacitors of this type has been limited by factors such as the limited minimum thickness of the dielectric material, which detracts from the flexibility of the capacitor and attainable capacitance, the effect of bond enhancers on the metal foils, low dielectric constant and poor dielectric strength. It is desirable to form a capacitor for a circuit board having a high dielectric constant and extremely thin layer or layers of dielectric material, thus increasing the capacitance and flexibility of the capacitor. However, a common problem frequently associated with such extremely thin dielectric layers is the formation of microscopic voids or other structural defects in the layer. For example, U.S. Pat. No. 5,161,086 provides a capacitor laminate having a single thin sheet of a dielectric material between two sheets of conductive foil. Dielectric layers of this type are highly vulnerable to the formation of voids and are time consuming to detect and remedy.
The present invention provides a capacitor that solves the problems of the prior art. The present invention provides capacitors that comprise a pair of parallel electrically conductive foils separated by a pair of coatings of a thermosetting polymer material, each preferably comprising an epoxy, and a central polymerizable layer. Each of the thermosetting layers and the central polymerizable layer preferably contains a filler material. In forming the capacitors of the invention, a liquid epoxy material is coated onto a surface of each of the conductive foils, followed by coating a central polymerizable layer in liquid form onto at least one of the epoxy coatings. The two coated conductive foils are then laminated together and the central layer is polymerized in-situ. The result is a multilayered structure having excellent properties such as high capacitance density, exceptional integrity and short circuit resistance. By incorporating a filler into at least one of these dielectric layers, the capacitance density of the capacitor may be controlled. Also, thin dielectric layers allow for greater heat conductivity and greater flexibility of the capacitor. The epoxy coatings and the central polymerizable layer further exhibit high resistance to thermal stress and low moisture absorption. Together, these factors offer a significant improvement in performance and cost over prior art capacitors and printed circuit boards.